Gonzalo Brito Gadeschi <g.brito <at> aia.rwth-aachen.de> writes:
I have a question: the proposal and library are called MPL11.
That is true, but I'm starting to realize that it's a very bad name. The project started about two years ago, when C++14 was still out of my sight. Be reassured that I always consider the latest version of the language, not only C++11.
In my experience, relaxed constexpr allows for beautiful metaprogramming with respect to C++11, in particular when combined with Boost.Fusion, and I think that it is really a game changer with respect to C++11 constexpr, since it allows not only "functional" metaprogramming but also "imperative" metaprogramming, significantly lowering the learning curve of metaprogramming for C++ programmers.
Could you please expand on the nature of metaprogramming you have been doing with constexpr? Are you talking about manipulating values of a literal type, or about "pure type" computations? See below for more on that.
Have you consider the influence of relaxed constexpr (C++14) in your library?
Could it simplify the design/implementation/usage of the MPL?
I did consider the impact of C++14 on the design of the library, and I still am. At this point, my conclusion is that we must define what we mean by a "template metaprogramming library". I differentiate between two main kinds of computations that can be done at compile-time. The first is manipulating "pure types" with metafunctions (e.g. type traits): using void_pointer = std::add_pointer_t<void>; using pointers = mpl::transform< mpl::list<int, char, void, my_own_type>, mpl::quote<std::add_pointer> >::type; The above example is completely artificial but you get the point. The second kind of computation is manipulating values of a literal type at compile-time. using three = mpl::plus<mpl::int_<1>, mpl::int_<2>>::type; The MPL wraps these values into types so it can treat computations on those as computations of the first kind, but another library could perhaps handle them differently (probably using constexpr). constexpr int three = plus(1, 2); It is easy to see how constexpr (and relaxed constexpr) can make the second kind of computation easier to express, since that is exactly its purpose. However, it is much less clear how constexpr helps us with computations of the first kind. And by that I really mean that using constexpr in some way to perform those computations might be more cumbersome and less efficient than good old metafunctions. As for using constexpr to express computations on literal values, another question arises. Should there be a general library for handling those at compile-time, or should it be the responsibility of a well written domain- specific library to provide means to perform domain-specific computations at compile-time whenever possible? For example, should the MPL11 provide constexpr math functions, or should the std:: math functions be constexpr whenever possible? Another example is std::array. While the MPL11 could provide a constexpr container for homogeneous values of a literal type, simply changing the `begin` and `end` of `std::array` to be constexpr would make it possible to iterate on the array at compile-time. template <typename T, std::size_t n> constexpr T sum(std::array<T, n> array) { T s{0}; for (T i: array) // awesome s += i; return s; } constexpr std::array<int, 5> array{{1, 2, 3, 4, 5}}; static_assert(sum(array) == 15, ""); Unfortunately, I don't think this is happening in C++14, but I could be mistaken. Still, you certainly understand the implications. So a valid question that must be answered before I/we can come up with a "final" version of the library that can be proposed to Boost (or for standardization) is: "What is the purpose of a TMP library?" Once that is well defined, we won't be shooting at a moving target anymore. Right now, I have avoided these questions as much as possible by focusing on computations of the first kind. For those computations, my research so far shows that constexpr is unlikely to be of any help. If someone can come up with counter-examples or ideas that seem to refute this, _please_ let me know and I'll even buy you a beer in Aspen. This is _very_ important; it's central to my current work.
Would a rewrite of MPL11 be necessary afterwards (MPL14)?
If constexpr turns out to be a game changer for computations of the first kind, then I suspect a large part of the current MPL11 would have to be rewritten. Of course, I tried to prevent this from happening by going slowly and looking at all the possibilities, but these things happen and it's part of the game. As for computations of the second kind, not _much_ work (comparatively) has been done on them because I knew it was a can of worms. So even drastic changes would not kill the library as it stands. Finally, if any of this does not make sense to someone, please let me know (on this list or privately) so I can explain further and correct any fallacious reasoning on my part. I hope this answers your questions; sorry for the long-winded answer but I think it can be informative for others too. Regards, Louis